Massive deposits formed in conditions that could not occur today.

The miners who answered the call of the California Gold Rush worked pretty hard in the hopes of striking it rich. But from a geological perspective, they were pretty lazy. Nature had already done most of the work. A lot of the gold they found was lying around in “placer” deposits—river sediment containing particles of solid gold. The gold originally came from igneous rocks, where it was very sparsely distributed among other minerals. Erosion liberated the gold and concentrated it, as water carried away lighter particles made of other minerals.

That’s one possible mechanism for gold to become plentiful enough that looking for it in a given volume of rock or sediment is economically viable. Another involves the movement of water heated by magma deep below the surface. The heated water dissolves and carries minerals, including gold, as it rises through rock. As it cools and moves through fractures, which form little highways, those dissolved minerals can precipitate out to form rich veins.

Some of the world’s best gold deposits can be found in South Africa’s Witwatersrand Basin. The Vaal Reef deposit, for example, is gigantic—it coughed up three thousand tons of gold, which would be worth well over $100 billion at today’s prices. However, geologists are still arguing about how the gold got there—riverine placer or hydrothermal precipitation are the two options mentioned above. The gold is found in layers within what was once sediment laid down by rivers, but these rivers flowed roughly three billion years ago. The deposits have become metamorphic rock in the intervening time.

Sounds like a placer deposit, right? Well, there are also clues that point to the gold having precipitated, which could have happened hundreds of millions of years later as hydrothermal fluids flowed through the deposits. So you have a case for hydrothermal deposition, too.

In a new paper published in Nature Geoscience, ETH Zürich geologist Christoph Heinrich outlines a third scenario that seems to explain almost all the observations. This alternative explanation relies on the fact that there was a very different environment three billion years ago—after the origin of microbial life but before the oxygenation of the atmosphere.

The hypothesis is built on the idea that voluminous eruptions of flood basalts that occurred in the area at the time would also have ejected a lot of volcanic gases—namely sulfur dioxide and hydrogen sulfide. Both those compounds would get into the river by riding in raindrops, with the sulfur dioxide forming sulfuric acid. The gold in these deposits is accompanied by a lot of fool’s gold (pyrite), which is composed of iron and sulfur, but very little of other iron-containing compounds. That would make sense if the water was loaded with sulfur, which grabbed the iron and then precipitated.

The volcanic rock that was weathering and eroding beneath the falling rain and flowing river water then would have contained diffuse gold. The acidic, low-oxygen, high-sulfur water it was exposed to during eruptions would have been good at dissolving gold—conditions you wouldn’t see today.

Downstream in quiet pools, that gold-laden water (at a concentration of maybe a part per billion) would encounter mats of living microbes, dead organic matter, or methane. Chemical reactions with that organic carbon would steal the gold atoms from their water-soluble partnership with hydrogen and sulfur, precipitating out the metal. And in fact, we do see what looks like gold precipitated onto microbial mats in these deposits.

After it precipitated, bits could break free and roll around, smoothing off rough edges. The end result would be something that looked a bit like a placer deposit and a bit like hydrothermal precipitation, which would explain why these rocks have been such a puzzle for geologists.

The volcanic eruptions weren’t constant, so the gold transport would have occurred in pulses. That massive Vaal Reef deposit, for example, could have accumulated over a million years, assuming a decade-long eruption every ten thousand years.

Heinrich does point out one observation that is problematic for his hypothesis: some trace element and isotopic data that suggests the particles of gold are older than the sedimentary rock it calls home. That shouldn’t be the case if the gold was precipitating in the way he’s proposing. However, there’s some uncertainty in those measurements that still needs to be worked out.

If the idea is right, this 3 billion year old bonanza only exists because of some special circumstances. An oxygen-free atmosphere and acid rain downwind of massive volcanic eruptions sounds pretty unfriendly to life like us, but unfazed microbial life snatched and hoarded the gold liberated by that rain. All we did was dig up their buried treasure.